MP6960 – FAST TURN-OFF INTELLIGENT CONTROLLER WITH CC/CV CONTROLLER
MP6960 Rev. 1.01 www.MonolithicPower.com 9
5/14/2012 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited.
© 2012 MPS. All Rights Reserved.
OPERATION
The MP6960 is a synchronous rectified controller
for use in constant-current mode (CCM),
discontinuous-current mode (DCM), and quasi-
resonant topologies. MP6960 integrates a high-
precision bandgap voltage reference, and two
opamps to regulate and control the output
voltage and current of the power supply.
Blanking
The control circuitry contains a blanking function
that ensures that when the MOSFET turns on or
off, it remains in that state for a minimal time
period. The turn on blanking time is ~1.6µs,
which determines the minimum on-time. During
the turn-on blanking period, the turn-off threshold
changes to approximately +50mV (instead of
-30mV) to ensure that the part can always turn
off even during the turn on blanking period (albeit
slower, so do not set the synchronous period to
less than 1.6µs in CCM for a flyback converter, or
risk shoot through).
Under-Voltage Lockout (UVLO)
When the VDD drops below the UVLO threshold,
the part enters sleep mode and the VG pin is
pulled low by a 10kΩ resistor.
Turn-On Phase
When the synchronous MOSFET is ON, the
current flowing through its body diode generates
a negative V
DS
. Because this body diode voltage
drop (<-500mV) is much smaller than the turn-on
threshold of the control circuitry (-70mV), then
MP6960 will then pull the gate driver voltage high
to turn on the synchronous MOSFET after a
150ns turn-on delay (Defined in Figure 2).
V
DS
V
GATE
t
Don
t
Doff
-70mV
-30mV
2V
Total
t
5V
Figure 2: Turn-On and Turn-Off Delay
Conducting Phase
When the synchronous MOSFET turns ON, V
DS
rises according to its ON resistance. As soon as
V
DS
rises above the turn-on threshold (-70mV),
the control circuitry stops pulling up the gate
driver, so the internal pull-down resistor (10k)
pulls down the gate voltage to increase the ON
resistance of the synchronous MOSFET to slow
the V
DS
rise. This adjusts V
DS
to around -70mV
even when the current through the MOSFET is
fairly small, and drop the driver voltage to fairly
low levels when the synchronous MOSFET turns
off to speed up turn-off (this driver-self-regulating
function is still active during the turn-on blanking
time so that the gate driver can turn off even with
a very small duty).
Turn-Off Phase
When V
DS
rises to trigger the turn-off threshold
(-30mV), the control circuitry causes the gate
voltage to go low after a 20ns turn-off delay (as
shown in Figure 2) by the control circuitry. A
200ns blanking time—similar to the turn-on
phase—after the synchronous MOSFET turns off
avoids erroneous triggering.
Figure 3 shows synchronous rectification at
heavy loads. The gate driver initially saturates
due to the high current. After V
DS
rises above
-70mV, gate driver voltage decreases to adjust
V
DS
to -70mV.
Figure 4 shows synchronous rectification under
light loads. Due to the low current, the gate driver
voltage never saturates but decreases as soon
as the synchronous MOSFET turns on and
adjusts V
DS
.
